According to recent technological trends for steering control, a control for a steering angle as well as a control for a power assisting steering have been incorporated in a vehicle, from a view point that a range for assisting vehicle travel will be enlarged to enhance comfort level, or that a range for supporting driver's intention will be enlarged to increase safety. A variable gear transmission system (VGTS) is known as one of those technologies, wherein a gear ratio between a steered angle of a vehicle wheel and a steering angle of a steering wheel is flexibly adjusted by an electric motor. Such system is generally incorporated in the vehicle together with an electrical power steering system, in which an auxiliary torque is generated by an electric motor to assist a steering operation of a vehicle driver. Such system is disclosed in, for example, Japanese Patent Publication Nos. 2001-287657 and 2005-247214.
In the steering system having the variable gear transmission system (VGTS) and the electrical power steering system, the electric motor for changing the gear ratio for the variable gear transmission system (VGTS) is independently provided in addition to the electric motor for generating the auxiliary torque for the electrical power steering system. As above, two control systems are provided for the common control subject (the steering system), which may cause the following problems.
At first, a mutual interference between two control systems may occur, because the control system for the variable gear transmission system (VGTS) and the control system for the electrical power steering system respectively control the same control subject (the steering system) by the respective electric motors. As a result, it is likely to cause deterioration of the operation sensitivity.
Secondly, the steering system may cause the deterioration of the operation sensitivity, because two independent electric motors are provided in the steering system, and thereby the moment of inertia which the vehicle driver could feel from the steering system will be increased. In particular, the moment of inertia of the electric motor for generating the auxiliary torque becomes larger and thereby the deterioration of the operation sensitivity will become marked, when the electrical power steering system is installed in a vehicle larger than a medium-sided vehicle having a large weight.